Adsorption 11: 145–149, 2005 c 2005 Springer Science + Business Media, Inc. Manufactured in The Netherlands.
Comparison of AgNO3 /Clay and AgNO3 /ALSG Sorbent for Ethylene Separation SOON-HAENG CHO∗, JONG-HO PARK, SANG-SUP HAN AND JONG-NAM KIM Separation Technology Research Center, Korea Institute of Energy Research, 71–2, Jang-dong, Yuseong-gu, Daejeon, 305–343, Korea
[email protected]
Abstract. Two π-complexation adsorbents, AgNO3 /clay and AgNO3 /ALSG, were prepared by dispersing AgNO3 on clay and aluminosilica substrate, respectively. Incipient wetness method was used in preparing the adsorbents. Adsorption capacities of AgNO3 /ALSG and AgNO3 /clay for ethylene at 25 o C and 1 atm were 1.81 mmol/g and 1.26 mmol/g, respectively. Binary adsorption measurement shows that the selectivity of AgNO3 /ALSG for ethylene at total pressure of 900 mmHg and 25o C was twice as high as that of AgNO3 /clay in the composition range of C2 gas mixture of practical importance. The performance of AgNO3 /ALSG has been compared with that of AgNO3 /clay by simulating a 3-bed VSA process. The simulation revealed that the recovery with AgNO3 /ALSG was higher by 4% at the purity of 99.9% than that with the AgNO3 /clay. Keywords: silver nitrate, adsorption, π -complexation, ethylene, pressure swing adsorption
1.
Introduction
Ethylene is the most important building block in any petrochemical industry. Steam cracking of naphtha or ethane is the main route of the ethylene production. In the cracked gas, a number of components are present. Separation of ethylene from the cracked gas is the essential step of ethylene production. The separations of ethylene from ethane and propylene from propane have been achieved conventionally by low temperature and/or high-pressure distillation. Therefore, the conventional process is one of the most energy intensive processes. For this reason, a number of researchers have been developing other separation processes, which can replace the distillation. Among the alternatives, separation based on π -complexation appears the most promising. The early attempts for the separation of olefin/ paraffin mixtures based on π -complexation employed liquid solutions containing silver (Ag+ ) or cuprous (Cu+ ) ions. Recently, efforts to find available sorbents ∗ To
whom correspondence should be addressed.
and to prepare new sorbents for the separations of ethylene/ethane and propylene/propane by pressure swing adsorption, have appeared in the literature. Especially, Yang et al. have prepared several kinds of new solid sorbents for selective light olefin over corresponding paraffin via π -complexation; Ag+ -exchanged resins (Yang et al., 1995), monolayered CuCl/γ -alumina, monolayered CuCl/pillared clays (Chen et al., 1995) and monolayered AgNO3 /SiO2 (Padin et al., 2000). Cho et al. (2001) prepared π -complexation adsorbent by dispersing AgNO3 on clay substrate. Though the adsorbent gave high adsorption capacity and selectivity for ethylene compared to the commercial adsorbents like zeolite, it showed adsorptiondesorption hysteresis so that the adsorption capacity of fresh adsorbent could not be fully utilized in cycle operation. To overcome the problem, new π -complexation adsorbent was prepared by dispersing AgNO3 on aluminosilica substrate (Son et al., 2003). The adsorbent which is called AgNO3 /ALSG showed higher adsorption capacity for ethylene than AgNO3 /clay. Furthermore, reversibility of adsorption-desorption was superior to AgNO3 /clay. However, since AgNO3 /ALSG
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Table 1.
Physical properties of substrates and prepared adsorbents. Average pore size (A)
Material
BET surface area (m2 /g)
Total mass balance: ∂ (uC) 1 − ε ∂ q¯ i ∂C + + ρp =0 ∂t ∂z ε ∂t i
Pore volume (cm3 /g)
Clay
40.3
392
0.42
AgNO3 /clay
34.0
172
0.23
ALSG
28.9
719
0.52
AgNO3 /ALSG
32.9
268
0.22
(2)
Energy balance for the gas phase includes the heat transfer to the column wall: ∂T ∂T + εc pg uC ∂t ∂z ∂ q¯ i 2h w − (−Ha )i (1 − ε)ρ p (T −Tw ) = 0 + ∂t Ri i
(εc pg C + (1 − ε)c ps ρ p ) adsorbed more ethane than AgNO3 /clay, the adsorption capacity ratios of ethylene to ethane of two sorbents were similar. In this study, performances of above two π-complexation adsorbents will be compared by simulation. Better sorbent for ethylene separation will be identified. 2.
Adsorption Preparation
Adsorbents compared in this study are prepared by dispersing AgNO3 on the clay and aluminosilica substrates. Incipient wetness method was utilized in preparing the π -complexation adsorbent. The detail recipe of preparation was found elsewhere (Cho et al., 2001). Average pore diameter, BET surface area and pore volume of the substrates and the corresponding π complexation adsorbents were given in Table 1. Average pore diameter of clay substrate is larger than that of aluminosilica substrate. After dispersing AgNO3 , the BET surface area and pore volume are reduced due to the blockage of the micropores. 3.
Mathematical Model
In Eq. (3), the last term accounts for the heat transfer to the column wall. Energy balance around the column wall is given by c pw ρw aw
(1)
∂ Tw = 2π h w Ri (T − Tw ) ∂t − 2πUw Ro (Tw − TF )
(4)
Mass transfer is expressed by the LDF approximation. ∂ q¯ i = ki (qi∗ − q¯ i ) ∂t
(5)
The steady state momentum balance is given by Ergun’s equation as follow: −
4.
The mathematical model adopted is a non-isothermal, non-equilibrium, bulk separation model. The assumption used to derive the model included the following: ideal gas behavior; no axial dispersion and no axial heat conduction; thermal equilibrium between gas phase and adsorbents. Mass transfer is represented by the linear driving force (LDF) approximation. Based on the above assumptions, the mass balance for each component of the mixture and the total mass balance are written as follows: Component mass balance ∂ yi ∂ yi (1 − ε) Rg T ∂ q¯ i +u + ρp ∂t ∂z ε P ∂t (1 − ε) Rg T ∂ q¯ j − yi ρp =0 ε P ∂t j
(3)
4.1.
∂P 1−ε 150µu (1 − ε)2 = + 1.75ρg u 2 2 2 ∂z dp ε dpε
(6)
Results and Discussion Pure Component Adsorption Isotherms
Pure component isotherms of ethane and ethylene on AgNO3 /clay and AgNO3 /ALSG at 25 o C are compared in Fig. 1. Adsorption isotherms of ethane and ethylene are fitted by the following models (Yang et al., 1995) and the results are shown in the figure as well. qsi bi pi (7) 1 + bi pi qsi bi pi qsc 1 + bc pi es (8) = + ln 1 + bi pi 2s 1 + bc pi e−s
qC∗ 2 H6 = qC∗ 2 H4
In Eq. (8), the second term of RHS represents the adsorption amount via π -complexation. Both adsorbents show high selectivity toward ethylene compared
Comparison of AgNO3 /Clay and AgNO3 /ALSG Sorbent
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Equilibrium models shown in Eqs. (9) and (10) are the simple extension of pure component adsorption isotherm. qC∗ 2 H6 = qC∗ 2 H4
Figure 1.
Adsorption isotherms of ethane and ethylene at 25◦ C
to the commercial adsorbents. As shown in the figure, AgNO3 /ALSG adsorbent has larger adsorption capacity for ethylene than AgNO3 /clay adsorbent. But, since AgNO3 /ALSG adsorbs more ethane than AgNO3 /clay, the adsorption capacity ratio, qC2H 4 /qC2H 6 , of two adsorbents at 1 atm are similar each other. 4.2.
Binary Adsorption Equilibrium
Binary adsorption equilibrium of ethylene and ethane was measured at 25◦ C and 900 mmHg, and the results are shown in Fig. 2. Lines shown in the figure are the predicted with the following models.
Figure 2. Binary adsorption equilibrium of ethane/ethylene at 25◦ C and 900 mmHg.
α · qsi bi pi 1 + j bj pj
(9)
1 + bc pi es α · qsi bi pi qsc ln = + 1 + j bj pj 2s 1 + bc pi e−s (10)
In case of ethane, for which physical adsorption is dominant, the extended Langmuir model was used with a correction factor, α. Pure component adsorption isotherm of ethane shown in Fig. 2 consists of the physical adsorption of ethane on both the silver nitrate and bare surface of substrate. Ethane adsorbed on silver nitrate surface is easily displaced by ethylene even when small amount of ethylene exists in the gas phase. α was introduced to account for the physical adsorption on the AgNO3 surface. Best fitting parameter of α was 0.5 for AgNO3 /ALSG and AgNO3 /clay. Selectivity for ethylene over ethane on the two adsorbents is shown in Fig. 3. It is clear from the figure that the AgNO3 /ALSG is more selective for ethylene than AgNO3 /clay especially in low ethane concentration. Since the source of ethylene/ethane mixture in industry contains high concentration of ethylene, AgNO3 /ALSG is better adsorbent in view of the selectivity and adsorption capacity.
Figure 3. Selectivity of two adsorbent for ethylene at 25◦ C and 900 mmHg.
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Table 2.
Cycle sequence of 3-bed VSA process.
Time(s) BED-1
100
5
45
AD
BED-2
EV
EQ
BED-3
Rinse
EQ
100
5
Rinse
EQ
BF
45
EV
AD EV
100
Rinse EQ
BF
5
45
EQ
BF
EQ
EV
AD
AD: adsorption step, BF: back fill step, EQ: pressure equalization step, EV: evacuation step.
4.3.
3-Bed VSA Process
A 3-bed VSA cycle performance with AgNO3 /clay by simulation was compared with AgNO3 /ALSG. Simulation was performed under the conditions that the adsorption and desorption pressures, and feed flow rate were 1050 mmHg, 20 mmHg and 3000 mL/min, respectively. The cycle sequence of the 3-bed VSA process is shown in Table 2. The performance curves shown in Fig. 4 were obtained varying the rinse flow rate. Therefore, higher purity of ethylene shown in the figure corresponds to the higher rinse flow rate. At relatively low purity of ethylene, about 99%, the recovery obtained with AgNO3 /clay is almost the same as that obtained with AgNO3 /ALSG. However, the superiority of AgNO3 /ALSG becomes clear as the purity of ethylene increases. At ethylene purity of 99.9%, the difference of recovery between AgNO3 /clay and AgNO3 /ALSG is about 4%. According to the simulation, ethylene purity of 99.9% can be produced with the recovery of 86% with AgNO3 /ALSG, and 82% with AgNO3 /clay.
Figure 5. Comparison of performance of AgNO3 /clay and AgNO3 /ALSG (Feed flow rates in two simulations are different).
Since the working capacity of AgNO3 /ALSG is higher than that of AgNO3 /clay, higher rinse flow rate is required to produce the same purity of ethylene with AgNO3 /ALSG as that with AgNO3 /clay. To produce the ethylene purity of 99.9% with AgNO3 /ALSG, rinse flow rate of 2200 ml/min is required, but with AgNO3 /clay 1100 ml/min is sufficient. To compare the performance of two adsorbents in terms of the productivity, simulation was performed for a 3-bed vacuum swing adsorption process using AgNO3 /ALSG adsorbent at different feed flow rate and compared the results to the AgNO3 /clay. The results are compared in Fig. 5. As shown in Fig. 5, the process using AgNO3 /ALSG gives similar performance even at higher feed flow rate. Since the cycle times of the two simulations are the same, the productivity of the process is solely determined by the feed flow rate and the recovery. Therefore, the productivity of the process with AgNO3 /ALSG is higher by 16% than that with AgNO3 /clay. Acknowledgments This research was supported by Korea Institute of Science & Technology Evaluation and Planning (KISTEP) under National Research Laboratory (NRL) program. References
Figure 4. Performance of 3-bed VSA process (PAD : 1050 mm Hg, PDE : 20 mm Hg).
Chen, J.P. and R.T. Yang, “Molecular Orbital Study of Selective Adsorption of Simple Hydrocarbons on Ag+ and Cu+ Exchanged Resins and Halides,” Langmuir, 11, 3450–3456 (1995).
Comparison of AgNO3 /Clay and AgNO3 /ALSG Sorbent
Cho, S.H., S.S. Han, J.N. Kim, N.V. Choudary, P. Kumar and S.G.T. Bhat, “Adsorbents, Methods for the Preparation and Method for the Separation of Unsaturated Hydrocarbons for Mixed Gases,” US Patent, 6315816 B1 (2001). Padin, J. and R.T. Yang, “New Aadsorbents for Olefin/Paraffin Separations by Adsorption via π -Complexation: Synthesis and Effects of substrates,” Chem. Eng. Sci., 55, 2607–2616 (2000).
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Son, Y.R., S.S. Han, J.H. Park, J.N. Kim, S.H. Cho, and T.J. Lee, “Study on the Adsorption Characteristics of Light Hydrocarbons on Aluminosilica Based Sorbent,” HWAHAK KONGHAK, 41(6), 749–755 (2003). Yang, R.T. and E.S. Kikkinides, “New Sorbents for Olefin/Paraffin Separations by Adsorption via π -Complexation,” AIChE J., 41(3), 509–517 (1995).
